Digital Subscriber Line (DSL) is a transmission technology combination in which copper telephone lines act as transmission media. The DSL includes High Rate Digital Subscriber Line (HDSL), Symmetric Digital Subscriber Line (SDSL), Very High Rate Digital Subscriber Line (VDSL), Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), etc., usually called as xDSL. There is a higher and higher XDSL access density, which is described by taking ADSL as an example. ADSL access is still used in high speed Internet access and in dedicated line services. Along with the growth of network video, distance learning, stock information transferring in real time and other multipoint communication services, providing multicast services by means of ADSL broadband access becomes an inevitable trend. In addition, along with a popular ADSL access application, more and more users need to access one access site, and a higher access density for an ADSL access apparatus is needed. Therefore high density will become a trend for the ADSL access apparatus.
In multicast services, the problems such as capture, identification, forwarding, management of multicast user, and right management of multicast user need to be solved principally. High access density in an ADSL apparatus is mainly implemented by cascading a slave block and a master block, and both the master block and the slave block implement the access function of an ADSL user. One master block may be cascaded with a plurality of slave blocks, and the ADSL user of the slave block accesses a network via the master block. The cascading of the slave block is a method for expanding ADSL access capacity. The master block is cascaded with the slave block via a cascaded interface board so as to implement the ADSL access of the slave block and to manage the slave block. Slave block multicast is needed because there is a need to support multicast services for ADSL users of the slave block. For the slave block multicast, how to conveniently manage multicast configuration data and efficiently implement multicast functions in real time are problems to be emphatically solved.
As shown in FIG. 1, in the prior art, the master block (i.e., master access module) and the slave block (i.e., slave access module) are connected with each other via an internal interface (the internal interface is an integrated channel for transferring unicast data, multicast data, and control information), access ADSL users respectively via a Remote Terminal Unit (RTU), and access a network via a network interface of the master block. The multicast message of ADSL user, accessed via the slave block, reaches the master block via Virtual Connection (VC) and is captured by the master block. The master block will identify the multicast message, authenticate the rights of multicast members, and forward multicast data to the VC of the multicast members. In addition, the configuration data of multicast is directly saved in the master block, and is not saved in the slave block. In this solution, the master block saves the configuration data of multicast and performs the multicast functions. The slave block only accesses the ADSL user and does not participate in the performing of multicast function. All the operation related to multicast (e.g., capture, explanation, authentication, and copying of multicast messages) is done by the master block. The slave block only provides a VC channel to enable the interaction between the ADSL users and the multicast module of the master block. The multicast control message (e.g., report message or leave message) is transferred from the ADSL user to the multicast server via the master block or the slave block and the multicast data message is transferred from the multicast server to the ADSL user via the master block or the slave block.
As shown in FIG. 2, the master block includes a protocol processing module, a multicast data forwarding module, a right management module, and a multicast member interface module. The protocol processing module is responsible for processing the IGMP multicast protocol and maintaining a multicast forwarding table; the multicast data forwarding module forwards multicast data to multicast members according to the multicast forwarding table. The right management module manages the rights of the multicast members and authenticates the multicast members. The multicast interface module manages multicast member interfaces. The slave block includes a multicast data forwarding module and a multicast member interface module. The multicast data forwarding modules of the master block and the slave block are connected with each other via cascaded lines.
The inventor of the present invention discloses following issues of the prior art in the design of the present invention.
1) The operation of the multicast member of a slave block is conducted in the master block, which results in a heavy burden on the master block and a low efficiency.
2) There is a serious waste in bandwidth. Duplicating the multicast data of the slave block by the master block has a high requirement on bandwidth resources of the master block. Multicast is a point to multipoint switching technique with a problem of a duplication point. In the prior art, the duplication point is in the master block. For example, if 100 ADSL users require joining one same group in the slave block, the master block will make 100 copies and send them to the slave block, and thus the bandwidth of 100 copies of multicast streams is consumed for the internal interfaces of the master block and the slave block, and for the internal bus of the master block.